The Sun newspaper paper acknowledged that the unidentified worker had been "properly vetted" and "thoroughly searched" once before entering the park on Friday morning but said he was later able to leave the park and re-enter without being challenged.
The tabloid claimed that he managed to take in a container that looked like a "dummy Semtex device" and trumpeted what it alleged was the discovery of a "terrifying flaw" in Olympic security.
"OLYMPIC SECURITY BLOWN APART," the paper's headline screamed.
Organizers were more measured.
"Clearly a Tupperware box containing batteries, a mobile phone and some loose wires would not pose a threat on a construction site," said a statement from the London Organizing Committee of the Olympic Games and Paralympic Games. "However we will be looking into the allegations to ensure that our security regimes are as robust as they need to be."
British tabloids routinely have reporters or others sneak into high-profile venues ahead of major events to highlight alleged security failings. In one of the most notorious examples, the Daily Mirror tabloid sent a reporter to work as a footman at Buckingham Palace ahead of then-president George W. Bush's visit to Britain.
Security is big issue ahead of the Olympic Games, which begin on July 27 and run until Aug. 12.